Diamine ligand release from the cisplatin analog [meso-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamine]dichloroplatinum(II) in cell culture medium

Abstract

The stability of the five-membered chelate ring of the cisplatin analog [meso-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamine]dichloroplatinum(II) (I) was investigated under typical cell culture conditions (IMEM-Richter's medium with 10% fetal calf serum, 37 Deg). The platinum compd. was radiolabeled with tritium in the meta position of the arom. ring by an acid-catalyzed tritium-exchange reaction, and a reversed-phase HPLC assay with radiochem. detection was developed to monitor for the presence of the free diamine ligand in the cell culture medium. A gradual increase in radioactivity attributed to the free diamine was found in medium contg. the dichloroplatinum(II) complex (ca. 25% after 24 h), indicating that the diamine ligand was being released from the metal atom. When 1 mM glutathione (GSH) was included in the incubation medium, the amt. of free diamine nearly doubled after 24 h, while the amt. of radioactivity attributed to serum protein-platinum adducts decreased relative to incubations without GSH. On the other hand, the omission of serum form the incubations resulted in a dramatic decrease in the amt. of radioactivity eluting under the diamine peak, while the concns. of the 2 methionine-Pt adducts, which formed in a 1:1 ratio, rose. Through the use of liq. secondary ion mass spectroscopy, the 2 methionine-Pt adducts were identified as monomethionine metabolites of the title compd., whereby the 2 chloride ligands were replaced by the amino acid. These compds. are probably diastereomers since the sulfur of methionine can coordinate to platinum with equal probability either cis or trans to the R-configured benzylamine carbon. On the basis of the chem. shifts of the MeS groups in the 250-MHz 1H NMR, it is concluded that a S,N-five-membered chelate ring is present in these methionine-Pt adducts.